Not if they're doing it intentionally, no. Cell phone companies do the same shit by offering unlimited data then decreasing the speed after a certain amount of data is used, and there was a big stink about it because it was false service. Now, if I were you, you should talk to your isp and tell them your issue and since most service calls are recorded, tell them on the record that you know it is illegal to offer and then knowingly restrict your bandwidth since you pay for such service.

Or your isp thinks you're a faggot.